We don't have an option to "un-share" a report in the client portal. However, there is a workaround that can give you similar results.

Step 1: Create a copy of your report.

Go to Reports > click Actions on the report you need > select Duplicate report

When a duplicate is created, the document is not shared

Step 2: Go back to the table of all reports.

Step 3: Once you have made sure you have the duplicate, you can delete the original report (which was shared).

You can differentiate them by checking the Shared column. The new one will be not shared. The old one will be marked as shared.